In an image forming apparatus employing an electro-photographic system, a static discharger employing a corona discharger such as a scorotron discharger is utilized in many cases. The static discharger employing a corona discharging system comprises discharger wires extending along the surface of a photo sensitive material, a shielding plate (rear plate) formed of a metal sheet so as to surround the discharger wires, and a mesh formed grid electrode (control electrode) provided between the discharging wires and the surface of the photo-sensitive material.
This static charger causes the discharging wires corona discharge by applying a high voltage (6K–8K volts) to the discharging wires and charges the photosensitive material with positive or negative electrons. Also the static charger is configured so as to adjust the static voltage of the photo-sensitive material by applying a bias voltage to the grid electrode.
It is known that in a corona discharge, much ozone is generated when electric charges are generated and has an adverse influence on image forming by remaining in the static discharger. For example, in an image forming apparatus employing an electro-photographic system, a rear plate is provided adjacent to the discharging wires in order to efficiently charge the photosensitive material. Consequently, it is know that generated ozone, etc. remains around the discharging wires in the static discharger which oxidizes the surface of the discharging wires and causes unevenness of electric charges during subsequent image forming operation.
In order to solve the above problems, several technologies are employed. Paragraph 0007 of Japanese Patent Open to Public (H07-334047) discloses a technology for efficiently removing ozone from a static discharger. Paragraph 0010 of Japanese Patent Laid Open to Public (H10-198238) and paragraph 0006 of Japanese Patent Open to Public (2000-365987) disclose a technology for blowing air into a static discharger so as not to adhere polluted air containing a suspended matter such as toner, etc. onto discharging wires. For example, Japanese Patent Open to Public (H07-334047) discloses an image forming apparatus featuring a duct with a dedicated air fan to exhaust air current into a corona discharger and an ozone disposal mechanism for guiding air current containing ozone to an ozone filter.
Japanese Patent Open to Public (H10-198238) discloses a technology to provide a static discharger by which discharge wires are uniformly oxidized by equalizing an air current blown in a shielding case along the longitudinal direction of the shielding case by providing a dedicated air duct and an air fan which exhaust most of the ozone being adhered onto the discharge wires. And Japanese Patent Open to Public (2000-365987) discloses an apparatus for removing ozone in an electrostatic charger by exhausting air within the electrostatic charger by providing an air-intake opening and an air-exhaust opening in an electrostatic charger employing a corona discharging system.
The structure of the image forming apparatus having a duct with a dedicated air fan for exhausting ozone by drawing air into an electrostatic charger is easily configured. However, it is a challenge to design a compact image forming apparatus and can be a main cost-up factor of the image forming apparatus. The importance of a blowing-air system is lowered in regard to a solution for the problem of a suspended matter such as toner and ozone remaining in an electrostatic charger adhered on discharge wires which cause unevenness of electrification since a method to prevent unevenness of electrification by sliding a cleaning mechanism (a wire cleaner) along the discharge wires has become a common practice.
The main purpose for blowing air into an electrostatic charger is not to removing ozone, etc. remaining in the electrostatic charger but to reduce image defects, such as image unevenness caused by ozone remaining in the electrostatic charger and to remove the factor preventing electrified charges from uniformly working on the surface of a paused photosensitive material. The effective means for blowing air to an electrostatic charger is not to continuously blow air to the electrostatic charger, but it is to blow enough air over a predetermined time after a copying operation. Continuously blowing air is not the optimal solution and the operation should be limited since continuously blowing air tends to foul the inside of the apparatus.